164 MR. R. J. TILLYARD ON THE RECTAL BREATHING-APPARATUS 



all forms o£ gill-baskefc, is the subject of a special section of this paper 

 (pp. 170-177). 



Shape of the Lamellce. — Text-fig. 14 gives us a very good idea of the 

 general shape of a typical lamella. The base is very broad, and the lamella 

 rises up very unsymmetrically from it. From the outer or distal end, 

 towards which the basal pad lies, the edge of the lamella rises up at a gentle 

 slant, which gradually increases towards the tip. But from the inner end the 

 lamella rises up much more sharply, forming a more regular oval curve. 

 The tip of the lamella may be evenly rounded, or somewhat " nodding " 

 towards the outer side. 



Tracheal Supply. — The distribution of primary and secondary efferent 

 tracheae to the gills is typically that described already for the Duplex System, 

 and need not be repeated here (see Plate 22. fig. 27). The tracheal supply of 

 the lamellae is, however, of great interest. Each lamella receives, beneath 

 its base, one large secondary efferent trachea, which breaks up at once into a 

 number of branches arising close together, nearly, but not quite, all at the same 

 level. From these branches there arise whole bundles or pencils of tracheal 

 capillaries, all again very nearly at the same level, close to and just within 

 the base of the lamella. Thus practically the whole of the lamellar area 

 contains capillaries only. The advantages of this arrangement also, for the 

 purpose of the extraction of oxygen, is so obvious as to need no comment. 



The capillaries run all very nearly parallel along the lamella until they 

 reach its distal end, where they all loop over inwards to return more along 

 the middle of the lamella, finally entering other bundles which join up to 

 form branches of the secondary efferent below the gill-base. This looping 

 of nearly all the capillaries inwards gives rise, in most cases, to a very 

 characteristic appearance of the capillaries just under the rounded distal end 

 of the lamella, an appearance which I propose to call the lamellar vortex. It 

 is most clearly seen in some genera of Corduliince (text-figs. 14, 15). The 

 in-turning of so many capillaries near the middle line of the gill gives a very 

 distinct appearance of a vortex of capillai'ies at this point. In those genera 

 of Lihellulince in which the tip of the lamella is very regular and rather flat^ 

 this voi'tex is not at all noticeable (Plate 21. fig. 13). 



I now propose to take the various genera in phylogenetic order, and indi- 

 cate the principal peculiarities in the gill-basket of each. 



(1) Synthemis and Metathemis. — The form of the gill-basket is almost 

 exactly the same in the larvae of S. eustalacta, S. macrostigma, and M. guttata. 

 This form may therefore be considered to be typical of the archaic tribe 

 Synthemini. The gill-basket is very small for the size of the larva, and is 

 found on eversion to be of a pure shining white colour without any trace of 

 pigmentation whatever. In each hemibranch there are only tivelve lamella?, 

 as against a number varying from twenty to thirty in all other genera of 

 Libellulidae. 



